Satellite telephone systems are emerging as a new and important global business. These systems utilize many individual circuits routed through one satellite or a constellation of many satellites to provide communications for terrestrial terminals. One significant advantage of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth without requiring the construction of many small terrestrial cells.
In the past, satellites have employed antennas that produce antenna beams to provide coverage to areas occupied by users or subscribers. For full duplex communication systems, it has been the practice for the beam pattern of the region covered by the antenna for transmission to be identical to the beam pattern of the region covered by the antenna for reception; i.e., the beam pattern of the regions for transmission is congruent with the beam pattern of the region covered for reception.
It has also been the practice to fill desired satellite service regions with more than one beam when there has been insufficient allocated bandwidth (frequency range) to serve a required number of users with a required bandwidth, and to lower the power requirements of both the satellites and the earth stations. This approach relies on the re-use of the allocated bandwidth with spatial diversity. Spatial diversity is established by dividing the service region into sub-regions, and employing separate and unique smaller beams or sub-beams to serve each service sub-region. This technique enables frequency re-use through spatial diversity, wherein adjacent sub-beams operate at different frequencies, and wherein non-adjacent sub-beams may use the same frequency, thereby avoiding interference between the users located in the non-adjacent sub-beams.
In the past, the transmission sub-regions (or sub-beams) have been defined to be the same shape and size as the corresponding reception sub-regions (or sub-beams). This is typically done in order to simplify signal routing and the overall system architecture. FIG. 1 depicts an example of a multibeam antenna beam pattern that can be used for both reception by user terminals (downlink) and transmission from user terminals (uplink). That is, the downlink beam pattern and the uplink beam pattern are substantially identical, or congruent, at the surface of the earth. In FIG. 1 the service region (SR) is partitioned into 16 sub-regions, individual ones of which are serviced by one sub-beam (assuming no sub-beam overlap). The pattern is characterized by a central sub-beam 1, a ring of six inner sub-beams (2-7), and a ring of nine outer sub-beams (8-16). Phased arrays are one suitable antenna type for generating such beam patterns. Reference in this regard can be had to, by example, U.S. Pat. No. 5,422,647, issued Jun. 6, 1995, entitled "Mobile Communication Satellite Payload", by E. Hirshfield and C. A. Tsao; U.S. Pat. No. 5,283,587, issued Feb. 1, 1994, entitled "Active Transmit Phased Array Antenna", by E. Hirshfield; and to U.S. Pat. No. 5,504,493, issued Apr. 2, 1996, entitled "Active Transmit Phased Array Antenna with Amplitude Taper", by E. Hirshfield.
More particularly, in the past satellites used for duplex communications have (as best as could be designed) congruent antenna beam coverage areas for a given uplink and downlink. For example, if a satellite has a certain coverage area and that coverage area is covered by sixteen separate beams (sub-beams) for the uplink and downlink, the sixteen beam coverage areas for the uplink are the same coverage areas for the downlink (with boundary lines located within, for example, 30 miles of one another), as shown in FIG. 1.
That is, in the conventional implementations of satellites used for duplex communications, the antenna beam area of the service links was designed to be the same for the uplink and the downlink. This was done for simplicity in satellite antenna design, simplicity in ground operations, and, when using conventional parabolic antennas, the antenna design optimization could be the same for the uplink frequency and the downlink frequency.
The inventors have realized that the use of fixed congruent sub-beams for transmission and reception, as in FIG. 1, can in some instances result in inefficiencies and a loss of overall system flexibility.